[GreenKeys] Loop driver question

[John Nagle]

> Date: Tue, 23 Apr 2013 20:39:19 -0500
> From: Ken Schwieker <ksweek at mindspring.com>
> To: GreenKeys at mailman.qth.net
> Subject: Re: [GreenKeys] Loop driver question
> 
> Thanks Eric for the web site and the reply. Now I need to figure out 
> if I can squeeze sufficient ma from the Tempest Dovetron output as is 
> or do I need to add a transistor stage inside. I presently don't have 
> the Dovetron or any paperwork at this location.
> 
> The previously mentioned transformer has a center tap so I could run 
> the PS at 50 volts DC out but am thinking that might start to get 
> close to marginal on range if used on radio reception. Might be an 
> interesting test to see if there is any range contraction in going to 
> 50 volts from 100 volts (each with the appropriate series resistance).
> 
> Ken S
> 
> At 05:44 PM 4/23/2013, you wrote:
>> >On Tue, Apr 23, 2013 at 10:34:33AM -0500, Ken Schwieker wrote:
>>> > >
>>> > > Wanted to find out if anyone has had experience with the CEL
>>> > > PS7141E-1A-A optocoupler used as a loop driver. Found it in a list
>>> > > on the following website.
>>> > > http://heepy.net/mediawiki/index.php/Optocouplers_for_teletype_current_loop
>>> > > Do not know who produced this website but thanks. Found that the
>>> > > PS7141 is available from Mouser at $1.56 each. Some of the others
>> >
>> >That's me :)
>> >
>> >The PS7141E is what I shipped with almost all of the 100+ little demod boards
>> >I was selling on here a while back. They work fine at normal TTY speeds. They
>> >would not be suitable for "modern" telecom speeds - the turn-on time 
>> >is hundreds
>> >of microseconds to milliseconds depending on LED drive current. 10 mA gets you
>> >under 1 ms which is sufficient for teletype. under 5mA they get very 
>> >very slow.  One nice side effect of them being slow is that that limits the 
>> > creation of inductive spikes :)

   The specs on that part look OK.  It's fast enough, has a high enough
current rating, and has a 400V breakdown voltage.

   I used to use Fairchild HSR-312L opto-isolators.  That part is now
obsolete.  Someone else used my board design with the Omron Electronics
G3VM-401B and reports it works fine at typical Baudot speeds.  All
three parts are 6-pin and have the same pinout.

   Some snubbing to damp inductive spikes down to below 400V is
indicated.  I just use a 1000pf 500V ceramic cap in series with a
100 ohm resistor across the selector magnet circuit.  The same
thing is done on the keyboard side.  This eliminates keyboard
contact arcing.

See: "http://www.aetherltd.com/connecting.html"

				John Nagle